Photoaddition reactions of N-benzylglycinates containing α-trimethylsilyl group with dimethyl acetylenedicarboxylate: competitive formation of pyrroles vs. β-enamino esters

Article abstract of DOI:10.1039/c8ra09996k

A study was conducted to gain insight into the preparative potential of photosensitized reactions of acyclic N-benzylglycinates containing an α-trimethylsilyl group with dimethyl acetylenedicarboxylate (DMAD). The photosensitizers employed in the reactions include 9,10-dicyanoanthracene (DCA), 1,4-dicyanonaphthalene (DCN), rose bengal (RB) and fullerene C₆₀. The results show that photoirradiation of oxygenated solutions containing the photosensitizers, glycinates and dimethyl acetylenedicarboxylate leads to competitive formation of pyrroles and β-enamino-esters. The distributions of pyrrole and β-enamino-ester products formed in these reactions are highly influenced by the electronic nature of the phenyl ring substituent on the benzylglycinates and the photosensitizer used. These photoaddition reactions take place via mechanistic pathways involving competitive formation of azomethine ylides and secondary amines, generated by a mechanistic routes involving initial SET from the benzylglycinates to photosensitizers.

Full text of DOI:10.1039/c8ra09996k

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Photoaddition reactions of N-benzylglycinates
containing a-trimethylsilyl group with dimethyl
acetylenedicarboxylate: competitive formation of
pyrroles vs. b-enamino esters†
Cite this: RSC Adv., 2019, 9, 5639
Suk Hyun Lim,a Amol B. Atar,a Gunoh Bae,b Kyung-Ryang Weeb
a
*
and Dae Won Cho
A study was conducted to gain insight into the preparative potential of photosensitized reactions of acyclic
N-benzylglycinates containing an a-trimethylsilyl group with dimethyl acetylenedicarboxylate (DMAD). The
photosensitizers employed in the reactions include 9,10-dicyanoanthracene (DCA), 1,4-
dicyanonaphthalene (DCN), rose bengal (RB) and fullerene C60. The results show that photoirradiation of
oxygenated solutions containing the photosensitizers, glycinates and dimethyl acetylenedicarboxylate
leads to competitive formation of pyrroles and b-enamino-esters. The distributions of pyrrole and b-
enamino-ester products formed in these reactions are highly influenced by the electronic nature of the
phenyl ring substituent on the benzylglycinates and the photosensitizer used. These photoaddition
reactions take place via mechanistic pathways involving competitive formation of azomethine ylides and
secondary amines, generated by a mechanistic routes involving initial SET from the benzylglycinates to
photosensitizers.
Received 5th December 2018
Accepted 2nd February 2019
DOI: 10.1039/c8ra09996k
rsc.li/rsc-advances
undergo ready secondary SET oxidation in the presence of
appropriate oxidants to form iminium ions 5.8a,17
Introduction
Among the interesting photochemical reactions that take
place through the sequential SET – a-CH deprotonation
pathway are those involving tertiary amines, which possess N-
carboxymethyl groups (i.e., glycinates).18 Tertiary glycinate
esters undergo unique cycloaddition reactions with alkenes/
alkynes, which take place via the intermediacy of 1,3-dipolar
azomethine ylides. An interesting example was uncovered in
early studies by Xiao18a and Rueping,18b which showed that
photoirradiation of solutions of ethyl 2-(3,4-dihydroisoquinolin-
2(1H)-yl)acetate derivatives 6 and electron decient alkenes/
Photoinduced single electron transfer (SET), occurring between
a variety of electron donating and accepting pairs, is now
recognized to be one of the most important events taking place
in mechanistically interesting and preparatively useful photo-
chemical reactions.1–6 Among
a wide variety of electron
donating substances that participate in these processes,
aliphatic/aromatic amines have perhaps been among the most
widely explored.7–10 In photoinduced SET reactions of aliphatic/
aromatic amines 1, aminium radicals 2 (i.e., amine radical
cations) serve as key reactive intermediates (Scheme 1). The
most common reaction pathways open to aminium radicals are
base-promoted deprotonation from either nitrogen to produce
either aminyl radicals 4 (in the cases of primary and secondary
aminium radicals)11,12 or a-carbon to form a-amino radicals 3
(in the case of tertiary aminium radicals).7,9b,13 The radical
intermediates generated in this manner take part in a variety of
addition reactions with electron decient alkenes, alkynes and
unsaturated carbonyl compounds.8,10a,12–15 In addition, owing to
their extremely low oxidation potential,16 a-amino radicals 3
aDepartment of Chemistry, Yeungnam University, Gyeongsan, Gyeongbuk 38541,
Republic of Korea. E-mail: dwcho00@yu.ac.kr
bDepartment of Chemistry, Daegu University, Gyeongsan, Gyeongbuk 38453, Republic
of Korea
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra09996k
Scheme
1 Photoinduced single electron transfer (SET) reaction
pathways opened for amine substrates.
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À
Á
3C*60 of the photosensitizer.20 Following the suggestion made
earlier by Foote and his coworkers,20c the formed 1O2 abstract H-
atom from the benzylic position of benzylglycinate 10 (E ¼ H),
lacking the a-trimethylsilyl group, followed by SET oxidation to
form iminium precursor of azomethine ylides 11, which then
cycloadd to C60 to form 12. However, in photoreactions of the a-
trimethylsilyl group containing N-benzylglycinate 10 (E ¼ SiMe3),
H-atom abstraction mediated by 1O2 takes place more efficiently
at the a-trimethylsilyl substituted carbon position to produce
azomethine ylides 13 that cycloadd to C60 to generate 14.
Although a variety of photosensitized 1,3-dipolar cyclization
reactions of either cyclic N-carboxyalkyl substituted tetrahy-
droisoquinolines21a or iminoester derivatives21b with dipolar-
ophiles have been mainly performed so far, to the best of
knowledge, cyclization reactions utilizing acyclic N-carboxyalkyl
substituted benzylamine substrates are not common. Thus, the
Scheme 2 SET-promoted 1,3-dipolar cycloaddition reactions of ethyl
2-(3,4-dihydroisoquinolin-2(1H)-yl)acetate 6 with dipolarophile.
alkynes, containing a photosensitizer and molecular oxygen, gives observations made in the investigation described above sug-
gested that azomethine ylides generated from acyclic N-benzyl-
glycinates containing a-trimethylsilyl group might participate in
photosensitized cycloaddition reactions with a variety of electron
decient dipolarophiles to produce ve membered ring N-
heterocycles. In order to assess this proposal, we explored pho-
rise to generation of pyrrolo[2,1-a]isoquinoline products 8
(Scheme 2). This process likely occurs through a mechanistic
pathway entailing deprotonation of an intermediate iminium ion
7 produced by a route (see Scheme 1) that begins with SET from 6
to the electronic excited state of the photosensitizer. The azome-
thine ylide intermediate 9, generated in this way, undergoes 1,3- tosensitized addition reactions of dimethyl acetylenedicarbox-
dipolar cycloaddition reactions with alkene/alkyne substrates.
ylate (DMAD) with N-a-trimethylsilyl-N-benzylglycinates,
In recent studies aimed at exploring photoaddition reactions possessing a variety of substituents on the arene ring of the
benzyl group. The photosensitizers (PS) employed in these
processes include 9,10-dicyanoanthracene (DCA), 1,4-dicyano-
naphthalene (DCN), rose bengal (RB) and fullerene C60. The
results arising from this effort show that two competitive path-
ways are followed in these reactions, one of which involves
of tertiary amines with fullerene C60, we also observed that N-
benzylglycinates 10 (E ¼ H) could serve as precursors of azome-
thine ylides (Scheme 3).10c,19 Specically, irradiation of solutions
of N-benzylglycinates 10 (E ¼ H) and C60 in the presence of
molecular oxygen gives rise to formation of pyrrolidine ring-fused
fullerene derivatives 12 (fulleropyrrolidines) (Scheme 3). In this cycloaddition to form pyrroles and the other generates b-
process, C60 serve as both the photosensitizer as well as reactive enamino-esters. In addition, the photoproduct distributions (i.e.,
dipolarophile. Moreover, we found that irradiation of oxygenated pyrrole/b-enamine ester ratios) are inuenced by the electronic
nature of the glycinate substrates and photosensitizer used.
solutions of C60 and N-benzylglycinates 10, which possess a-tri-
methylsilyl groups (E ¼ SiMe3), promotes generation of trime-
thylsilyl group containing fulleropyrrolidines 14 in a much more
efficient manner.19 These observations suggested that the routes
for formation of these products begin with the generation of
singlet oxygen (1O2) via energy transfer from the triplet state
Results and discussion
N-a-Trimethylsilyl-N-benzylglycinates 16a–16g, in which the
benzyl moieties contain various substituents on the phenyl ring,
Scheme 3 SET-promoted 1,3-dipolar cycloaddition reactions of tertiary N-benzylglycinates 10 (E ¼ H, SiMe3) with fullerene C60
.
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Table
1
Oxidation potentials (Eox), free energy changes for SET
were prepared using an earlier developed synthetic proto-
col10c,19,22 (Scheme 4). Photochemical reactions were carried out
using O2-purged MeCN or toluene solutions containing 16a–16g
(3.2 mM) and DMAD (17, 3.2 mM) in the presence of SET
photosensitizer. The solutions were irradiated for time periods
that bring about 100% conversion of the starting glycinates by
using a 450 W Hanovia mercury lamp equipped with a glass
lter (l > 310 nm). In all cases, the photolysates were concen-
trated and the residues were subjected to column chromatog-
raphy to produce pure samples of the respective photoproducts.
(DGSET) and rate constants for DCA fluorescence quenching by N-a-
trimethylsilyl-N-benzylglycinates 16a–16g in MeCN solutions
Eox(+)
(V vs. SCE)
kSET ꢀ 10ꢁ10
(Mꢁ1 sꢁ1
)
a
Glycinate
DGSET (V)
16a
16b
16c
16d
16e
16f
0.84
0.83
0.83
0.84
0.80
0.89
0.91
ꢁ1.15
ꢁ1.16
ꢁ1.16
ꢁ1.15
ꢁ1.19
ꢁ1.10
ꢁ1.08
1.1
1.0
b
—
—
b
0.99
b
—
16g
0.86
DCA-promoted photochemical reactions
a
Determined by using DGSET ¼ Eox (glycinate) ꢁ Ered (DCA) ꢁ Eex (DCA),
where Ered (DCA) is ꢁ0.91 V (vs. SCE) (ref. 23) and Eex (DCA) is 2.90 V (ref.
23). b Not measured.
Prior to carrying out DCA promoted photochemical reactions,
oxidation potentials (Eox) of the glycinate substrates 16a–16g
were determined in order to ascertain whether SET from the
glycinates to the singlet excited state of the SET photosensitizer
DCA (1DCA) is thermodynamically favorable. In addition, the Table 2 Products and yields of DCA-photosensitized reactions of
oxygen purged MeCN solutions containing glycinates 16a–16g and
rate constants for quenching of the uorescence of DCA by
glycinates 16a–16g in MeCN were determined in order to derive
acetylene 17a
the rate constants (kSET) for SET. As can be seen viewing the data
Irradiation
time (min)
Entry
Amine
X
Productb (%)
in Table 1, the glycinates have Eox values in the range of 0.83–
0.91 V (vs. SCE), which are below the reduction potential of
singlet state of DCA (1DCA) (1Ered(DCA) ¼ 1.99 V vs. SCE). As
a result, free energy changes for SET (DGSET) from all glycinates
to 1DCA are negative, suggesting that the rates of these
processes should be near the diffusion controlled limit. Stern–
Volmer analysis of plots of uorescence intensities of DCA vs.
glycinate concentrations (Fig. S1, ESI†), showed that the rates of
uorescence quenching by SET from the glycinates 16a–16g to
1DCA are near that of diffusion in MeCN.
Photochemical reactions of N-benzylglycinates 16a–16g with
DMAD (17) were performed in O2-purged MeCN solutions in the
presence of DCA for the time periods given in Table 2.
Concentration of the photolysates followed by chromatographic
separation gave the products shown in Scheme 5 and the yields
listed in Table 2. As can be seen by viewing the results, 5 min
irradiation of O2-purged MeCN solutions containing DCA, acet-
ylene 17 and glycinates 16a–16d, which possess H and electron
donating groups (Me, OMe) on the phenyl ring of the benzyl
substituent, gave rise to production of the respective b-enamino-
esters 19a–19d, along with minor amounts of the corresponding
substituted pyrroles 18a–18d (entries 1–4, Table 2). In contrast,
1
2
3
4
5
6
16a
16b
16c
16d
16e
16f
16g
16a
16a
H
5
5
5
18a (6), 19a (50)
18b (4), 19b (51)
18c (trace), 19c (48)
18d (trace), 19d (45)
18e (21), 19e (31)
18f (24), 19f (33)
18g (32), 19g (22)
n.rd
p-Me
o,p-di-Me
p-OMe
p-F
o,p-di-F
p-CF3
H
5
10
10
20
300
300
7
8c
9e
H
n.rd
a
220 mL of MeCN solutions containing glycinates 16a–16g (3.2 mM),
acetylene 17 (3.2 mM) and DCA (0.27 mM).
Photoreaction in N2-purged MeCN solution.
Photoreaction in O2-purged MeCN solution without DCA.
b
Isolated yields.
No reaction.
c
d
e
10 min irradiation of solutions containing the mono- and di-
uoro substituted phenyl containing glycinates 16e–16f, 17 and
DCA produced ca. 1 : 1.5 ratios of pyrroles (18e–18f) and b-
enamino-esters (19e–19f) (entries 5 and 6, Table 2). Finally, DCA-
promoted photoreaction of the para-CF3-phenyl containing gly-
cinate 16g with 17 took place to produce both trimethylsilyl-
substituted pyrrole 18g (32%) and b-enamino-ester 19g (22%)
by 20 min irradiation (entry 7 in Table 2). Importantly, the
concentration of DCA in all photoreactions was kept constant
and control experiments (entries 8 and 9 in Table 2) revealed that
both molecular oxygen and DCA are necessary for reactions to
take place.
DCN-promoted photochemical reactions
DCN was also utilized as a SET photosensitizer to promote
reactions of the glycinates with acetylene 17. Similar to the case
of DCA, the calculated free energy changes (DGSET) for SET from
the glycinates to the singlet state of DCN (1Ered ¼ 2.3 V vs. SCE)23
were negative. In addition, the results of uorescence quench-
ing experiments showed that the glycinates quench the singlet
Scheme 4 Synthesis of N-a-trimethylsilyl-N-benzylglycinates 16a–
16g.
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Scheme 5 DCA-promoted photochemical reactions of N-benzylglycinates 16a–16g with DMAD 17.
state of DCN (1DCN) with near diffusion controlled rates (Fig. S2
of ESI†).
The results displayed in Table 4 show that RB-promoted
photochemical reactions O2-purged MeCN solutions of glyci-
The results of photochemical reactions of N-a-trimethylsilyl- nates 16a, 16b, 16e and 16g with acetylene 17 take place effi-
N-benzylglycinates 16a, 16b, 16e and 16g with acetylene 17 in ciently to produce pyrroles and b-enamino-esters. Specically,
O2-purged MeCN solutions in the presence of DCN are shown in 5 min irradiation of RB solutions containing 16a–16b and 17
Table 3. As the data show, the DCN-promoted photoreactions produced a ca. 1 : 4 ratios of the corresponding pyrroles 18a–
took place to produce pyrroles and b-enamino-esters but, in 18b (11–12%) and b-enamino-esters 19a–19b (47–49%) (entries
contrast to the DCA-photosensitized reactions, they required 1 and 2 in Table 4). Interestingly, in contrast to those photo-
much longer irradiation times to bring about complete sensitized by DCA, RB-promoted reactions of the p-F- (16e) and
conversion of glycinate substrates. For example, 60 min irradi- p-CF3- (16g) phenyl substituted glycinates with 17 produced
ation of an oxygenated MeCN solution containing 16a, 17 and higher yields of the respective b-enamino-esters 19e and 19g
DCN gave rise to formation of b-enamino-ester 19a (40%), along (41–45%) than pyrroles 18e and 18g (11–12%) (entries 3 and 4 in
with pyrrole 18a (11%) (entry 1 in Table 3). Moreover, 60 min Table 4).
photochemical reaction of the p-Me-phenyl substituted glyci-
nate 16b and 17 in the presence of DCN produced b-enamino-
C
60-promoted photochemical reactions
ester 19b exclusively (entry 2 in Table 3), and photoreactions
of the p-F- (16e) and p-CF3-substituted (16g) glycinates with 17
occurred to produce ca. 1 : 3 and 2 : 3 respective ratios of
pyrroles (18e and 18g) and the respective enamino-esters (19e
and 19g) by much longer irradiation (entries 3 and 4 in Table 3).
A consideration of redox potentials suggests that C60 (3Ered
¼
1.14 V vs. SCE)25 should serve as a SET-photosensitizer for
reactions between the glycinates and DMAD. Owing to the
generally low solubility of fullerene C60 in MeCN, toluene was
used as the solvent for C60-promoted photoaddition reactions of
N-a-trimethylsilyl-N-benzylglycinates 16a–16g with acetylene 17.
Inspection of the results summarized in Table 5 showed that the
product distributions patterns arising from these photochem-
ical reactions are comparable with those from DCA-, DCN- and
RB-promoted photochemical reactions of these substrates.
Specically, irradiation of O2-purged toluene solutions con-
taining glycinates 16a–16g, 17 and C60 produced pyrroles 18a–
RB-promoted photochemical reactions
RB-promoted photoaddition reactions of N-a-trimethylsilyl-N-
benzylglycinates with acetylene 17 were also investigated.
Similar to the cases of DCA and DCN, the free energy changes
for SET occurring between glycinates and RB (S1Ered ¼ 1.18 V,
T1
E
¼ 1.02 V vs. SCE)24 are all negative.
red
Table 4 Products and yields of RB-photosensitized reactions of O2-
purged MeCN solutions containing glycinates (16a, 16b, 16e and 16g)
and acetylene 17a
Table 3 Products and yields of DCN-photosensitized reactions of O2-
purged MeCN solutions containing glycinates (16a, 16b, 16e and 16g)
and acetylene 17a
Irradiation
time (min)
Entry
Glycinate
Productb (%)
Irradiation
time (min)
Entry
Glycinate
Productb (%)
1
2
16a
16b
5
5
18a (11), 19a (49)
18b (12), 19b
(47)
18e (12), 19e (45)
18g (11), 19g (41)
1
2
3
4
16a
16b
16e
16g
60
60
90
18a (11), 19a (40)
18b (4), 19b (49)
18e (12), 19e (40)
18g (21), 19g (32)
3
4
16e
16g
10
30
180
a
a
220 mL of MeCN solutions containing glycinates (16a, 16b, 16e and
220 mL of MeCN solution containing glycinate (3.2 mM), acetylene (3.2
mM) and RB (0.32 mM). b Isolated yields.
b
16g, 3.2 mM), acetylene 17 (3.2 mM) and DCN (0.32 mM). Isolated
yields.
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Table 5 Products and yields of C60-photosensitized reactions of O2-
purged toluene solutions containing glycinates 16a–16g and acety-
lene 17a
were produced and no pyrrole products are generated (entries
1–4 in Table 6). In contrast, photoreactions of non-ester group
substituted amine 21 with 17 gave rise to b-enamino-ester 23 as
a single photoadduct (entries 5–8 in Table 6). Thus, it appears
that the presence of both the trimethylsilyl and ester groups in
the N-benzylamine substrates are essential for these SET-
photosensitized reactions to produce pyrrole adducts.
Irradiation
time (min)
Entry
Glycinate
Productb (%)
1
2
3
4
16a
16b
16c
16d
10
10
10
10
18a (38), 19a (21)
18b (41), 19b (19)
18c (38), 19c (16)
18d (39), 19d
Mechanistic pathways
(18)
5
6
7
16e
16f
16g
20
20
30
18e (35), 19e (20)
18f (32), 19f (21)
18g (31), 19g (21)
The SET-photosensitized reactions described above most likely
follow the pathways outlined in Scheme 6, which are terminated
either by Michael addition of secondary amines 37 to DMAD to
generate b-enamino-esters 40 or by dipolar cycloaddition of
azomethine ylides 36 to DMAD to produce precursors of the
pyrroles 39. In these processes, the secondary amines (37) and
ylides (36) could be formed from the N-a-trimethylsilyl-N-benzyl
glycinates (GL) through a number of different routes. However,
it is nearly certain that the b-enamino-ester and pyrrole forming
photoreactions are both initiated by SET from the glycinates to
the excited states of the sensitizers (S1 or T1Sens). This proposal
is based on the results of the uorescence quenching studies
described above and a consideration of the concentrations of
the glycinates used. Accordingly, the rates of the SET processes
(kSET [glycinate], where kSET ¼ kdiff ¼ ca. 1 ꢀ 109 Mꢁ1 sꢁ1 and
a
220 mL of toluene solution containing glycinate (3.2 mM), acetylene
(3.2 mM) and C60 (016 mM). b Isolated yields.
18g as major photoproducts and b-enamino-esters 19a–19g as
minor products (Table 5). Noticeably, no photoproduct arising
from addition reactions between glycinates and C60 were
observed.19
N-trimethylsilyl- and ester-substituent effects on photochemical
reactions
In order to gain additional information about how N-substitu- [glycinate] ¼ 3.2 ꢀ 10ꢁ3 M) should far exceed those for excited
ents on the glycinates (e.g., trimethylsilyl and ester groups) state sensitizer decay and energy transfer to molecular oxygen
inuence the efficiencies and product distributions, we probed (kET [O2], where kET ¼ ca. 1.9–7.5 ꢀ 109 Mꢁ1 sꢁ1 and [O2] ꢂ
SET photosensitized reactions of acetylene 17 with the non- [glycinate]).20a,27
S1 or
trimethylsilyl and non-ester group containing N-benzylamine
SET from GL to
T1Sens in the mechanistic pathway
substrates, 20 (ref. 19) and 21,10c respectively. As the results generates a glycinate derived aminium radical 32 and a radical
depicted in Table 6 show, in the photoreactions of non- anion of the photosensitizer (Senscꢁ), the former of which
trimethylsilyl substituted amine 20 with acetylene 17, serves as a key intermediate in the pathways that produce the
mixtures of several types of b-enamino-esters, 19a, 22 and 23,26 pyrrole and b-enamino-ester products. The generated radical
Table 6 Products and yields of SET-photosensitized reactions of N-benzylamines 20–21 with acetylene 17a
Entry
Amine
Reaction condition
Irradiation time (min)
Productb (%)
1
2
3
4
5
6
7
8
20
20
20
20
21
21
21
21
DCA in MeCN
DCN in MeCN
RB in MeCN
C60 in toluene
DCA in MeCN
DCN in MeCN
RB in MeCN
10
60
10
30
5
60
5
10
19a (10), 22 (34), 23 (10)
19a (14), 22 (32), 23 (10)
19a (17), 22 (29), 23 (11)
19a (42), 22 (19)
23 (61)
23 (60)
23 (58)
23 (78)
C60 in toluene
a
220 mL of MeCN or toluene solutions containing N-benzylamines (3.2 mM), acetylene (3.2 mM) and photosensitizer (DCA (0.27 mM), DCN (0.32
mM), RB (0.32 mM) and C60 (0.16 mM)). b Isolated yields.
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Scheme 6 Proposed mechanistic pathways.
3
anion of the photosensitizer is oxidized by O2 to ground state on arene ring, with electron decient dimethyl acetylenedi-
of photosensitizer (Sens) and O2cꢁ is generated at the same carboxylate (DMAD) in the presence of various kinds of SET-
time. While a number of pathways are possible for conversion of photosensitizers were explored in order to assess the prepara-
32 to secondary amine 35 and ylide 34, an initial route involving tive utility of trimethylsilyl group containing acyclic N-benzyl-
O2cꢁ promoted H-atom abstraction of the aminium radical 32 glycinates as substrates in azomethine ylide dipolar
(leading to 33) is most plausible based on observations made in cycloaddition reactions.
earlier studies.18,28 Then, generated silyl containing iminium
The results showed that two competitive pathways are fol-
ion 33 undergo hydrolytic cleavage by hydrogen peroxide anion lowed in these reactions, one of which involves cycloaddition to
(HOOꢁ) to form secondary amine 35, which add to DMAD to form pyrroles via azomethine ylides and the other generates b-
yield b-amino-ester 37. Competitively, deprotonation of silyl enamino-esters via secondary amines. Importantly, the photo-
group containing iminium ion 33 produces azomethine ylide product distributions (i.e., pyrrole/b-enamine ester ratios) are
34, which cycloadd to DMAD, followed aromatization, to form inuenced by the electronic nature of the glycinate substrates
pyrrole 36.
While conforming to observations made in this study, it is
and photosensitizer used.
Finally, a plausible mechanistic pathway has been proposed
difficult to explain the regioselectivity for H-atom abstraction for SET-promoted oxidation reactions of tertiary amines. This
process from 32 (leading to iminium ion 33) and, in particular, proposal along with other interesting features of these reactions
why loss of benzylic or a-ester hydrogen do not take place will guide the design of future mechanistic and preparative
competitively. However, it is clear that in contrast to non- studies.
regioselectivity of aminium radical 38 derived from non-silyl
amine analog 21 (Scheme 7), a presence of electrofugal group
(i.e., SiMe3) in the aminium radicals 32 might lead to regiose-
lective H-atom abstraction process.
Experimental
General
The 1H (300 MHz) and 13C NMR (75 MHz) spectra were recorded
on CDCl3, and chemical shis were reported in parts per
million (d, ppm) relative to CHCl3 (7.24 ppm for 1H and
Conclusion
In this study, photoaddition reactions of N-a-trimethylsilyl-N- 77.0 ppm for 13C) as an internal standard. High resolution
benzylglycinates, which contain various kinds of substituents (HRMS) mass spectra were obtained by use of quadrupole mass
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Scheme 7 Non-regioselective formation of iminium ions 39–41 from non-silyl amine analog 21.
analyzer and electron impact ionization unless otherwise noted. 162.0 (dd, J ¼ 245.9 Hz, 12 Hz), 171.1; HRMS (EI) m/z 315.1465
All new compounds described were isolated as oils unless noted (M+, C15H23F2NO2Si requires 315.1466).
otherwise.
General procedure of photoreactions of N-a-trimethylsilyl-N-
benzylglycinates and dimethyl acetylenedicarboxylate (DMAD)
Synthesis of N-a-trimethylsilyl-N-benzylglycinates 16a–16g
in the presence of photosensitizer
Individual solutions of N-a-trimethylsilyl-N-benzylamines 15a–
15g (ref. 22a) (10 mmol) in acetonitrile (100 mL) containing
K2CO3 (42 mmol) and ethyl bromoacetate (30 mmol) were stir-
red for 12 h at room temperature and concentrated in vacuo to
give residues that were triturated with CH2Cl2. The triturates
were dried and concentrated in vacuo to afford residues, which
were subjected to silica gel column chromatography (EtOAc/
hexane ¼ 1 : 5 to 1 : 8) to yield 16a (ref. 10c) (70%), 16b (ref.
19) (66%), 16c (66%), 16d (ref. 19) (51%), 16e (ref. 19) (74%), 16f
(68%) and 16g (ref. 19) (55%) respectively.
Preparative photochemical reactions were conducted using an
apparatus consisting of a 450 W Hanovia medium vapor pres-
sure mercury lamp equipped with a int glass lter (>310 nm) in
a water-cooled quartz immersion well surrounded by the solu-
tion being irradiated, consisting of solution (220 mL) contain-
ing glycinate (0.7 mmol, 3.2 mM), acetylene 17 (0.7 mmol, 3.2
mM), and photocatalyst (DCA (0.27 mM), DCN (0.32 mM), RB
(0.32 mM), C60 (0.16 mM)). The solution being irradiated was
purged with oxygen before and during irradiations for the time
periods given below. The photolysates were concentrated in
vacuo to yield residues, which were subjected to silica gel
column chromatography to isolate the pure photoproducts.
16c (yellow liq). 1H-NMR 0.05 (s, 9H), 1.26 (t, 3H, J ¼ 6.9 Hz),
2.24 (s, 2H), 2.30 (s, 3H), 2.34 (s, 3H), 3.25 (s, 2H), 3.75 (s, 2H),
4.14 (q, 2H, J ¼ 6.9 Hz), 6.95 (d, 1H, J ¼ 7.5 Hz), 6.96 (s, 1H), 7.19
(d, 1H, J ¼ 7.5 Hz); 13C-NMR ꢁ1.6, 14.2, 19.0, 20.9, 45.5, 56.4,
59.8, 126.0, 129.8, 130.9, 134.0, 136.4, 137.3, 171.3; HRMS (EI)
m/z 307.1965 (M+, C17H29NO2Si requires 307.1968).
Photoreactions of oxygenated solution of 16a and 17
16f (yellow liq). 1H-NMR 0.01 (s, 9H), 1.22 (t, 3H, J ¼ 7.2 Hz), In MeCN solution of DCA: 5 min irradiation, column chroma-
2.16 (s, 2H), 3.22 (s, 2H), 3.73 (s, 2H), 4.12 (q, 2H, J ¼ 7.2 Hz), tography (EtOAc : hexane ¼ 1 : 5) to yield 18a (18 mg, 6%) and
6.68–6.75 (m, 1H), 6.76–6.82 (m, 1H), 7.36–7.43 (m, 1H); 13C- 19a (117 mg, 50%). In MeCN solution of DCN: 60 min irradia-
NMR ꢁ1.7, 14.2, 45.6, 53.6, 57.0, 60.1, 103.4 (t, J ¼ 25.5 Hz), tion, column chromatography to yield 18a (32 mg, 11%) and 19a
110.9 (dd, J ¼ 20.8 Hz, 3.8 Hz), 121.9 (dd, J ¼ 14.3 Hz, 3.6 Hz), (94 mg, 40%). In MeCN solution of RB: 5 min irradiation,
131.8 (dd, J ¼ 9.4 Hz, 6.2 Hz), 161.2 (dd, J ¼ 246.9 Hz, 11.8 Hz), column chromatography to yield 18a (32 mg, 11%) and 19a
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(115 mg, 49%). In toluene solution of C60: 20 min irradiation, 129.0, 131.5, 136.7, 138.0, 154.4, 165.7, 167.8, 168.5; HRMS (EI)
column chromatography to yield 18a (111 mg, 38%) and 19a m/z 363.1680 (M + 1, C19H25NO6 requires 363.1682).
(49 mg, 21%).
18a (yellow liq). 1H-NMR 0.26 (s, 9H), 1.19 (t, 3H, J ¼ 7.2 Hz), Photoreactions of oxygenated solution of 16d and 17
3.80 (s, 3H), 3.88 (s, 3H), 4.12 (q, 2H, J ¼ 7.2 Hz), 5.77 (s, 2H),
In MeCN solution of DCA: 5 min irradiation, column chroma-
6.78 (d, 2H, J ¼ 7.2 Hz), 7.18–7.31 (m, 3H); 13C-NMR 1.2, 14.0,
tography (EtOAc : hexane ¼ 1 : 5) to yield 18d (3 mg, 1%) and
51.5, 51.8, 52.6, 61.1, 122.7, 124.4, 125.2, 126.7, 127.3, 128.8,
138.4, 145.6, 159.6, 164.4, 166.9; HRMS (FAB) m/z 418.1680 (M +
1, C21H28NO6Si requires 418.1686).
19d (115 mg, 45%). In toluene solution of C60: 10 min irradia-
tion, column chromatography to yield 18d (123 mg, 39%) and
19d (46 mg, 18%).
19a (yellow liq). 1H-NMR 1.23 (t, 3H, J ¼ 6.9 Hz), 3.59 (s, 3H),
3.70 (s, 2H), 3.89 (s, 3H), 4.16 (q, 1H, J ¼ 6.9 Hz), 4.38 (s, 2H),
4.70 (s, 1H), 7.22–7.34 (m, 5H); 13C-NMR 14.2, 50.2, 51.1, 53.2,
55.2, 61.7, 87.0, 128.1, 128.3, 129.0, 134.9, 154.5, 165.9, 167.9,
168.3; HRMS (FAB) m/z 336.1445 (M + 1, C17H22NO6 requires
336.1447).
18d (yellow liq). 1H-NMR 0.27 (s, 9H), 1.19 (t, 3H, J ¼ 7.2 Hz),
2.28 (s, 3H), 3.74 (s, 3H), 3.78 (s, 3H), 3.86 (s, 3H), 4.12 (q, 2H, J ¼
7.2 Hz), 5.68 (s, 2H), 6.70 (d, 2H, J ¼ 8.7 Hz), 6.78 (d, 2H, J ¼ 8.7
Hz); 13C-NMR 1.0, 13.8, 50.8, 51.6, 52.3, 55.1, 60.9, 114.0, 122.4,
124.2, 126.2, 126.3, 130.1, 145.2, 158.6, 159.4, 164.2, 166.6;
HRMS (EI) m/z 447.1717 (M + 1, C22H29NO7Si requires
447.1713).
19d (yellow liq). 1H-NMR 1.23 (t, 3H, J ¼ 7.2 Hz), 3.61 (s, 3H),
3.67 (s, 2H), 3.77 (s, 3H), 3.90 (s, 3H), 4.16 (q, 1H, J ¼ 7.2 Hz),
4.31 (s, 2H), 4.70 (s, 1H), 6.84 (d, 2H, J ¼ 8.7 Hz), 7.17 (d, 2H, J ¼
8.7 Hz); 13C-NMR 13.8, 50.6, 52.7, 54.2, 54.9, 61.2, 86.2, 113.9,
126.3, 128.9, 129.2, 154.1, 159.2, 165.5, 167.5, 167.9; HRMS (EI)
m/z 365.1477 (M + 1, C18H23NO7 requires 365.1475).
Photoreactions of oxygenated solution of 16b and 17
In MeCN solution of DCA: 5 min irradiation, column chroma-
tography (EtOAc : hexane ¼ 1 : 5) to yield 18b (12 mg, 4%) and
19b (125 mg, 51%). In MeCN solution of DCN: 60 min irradia-
tion, column chromatography to yield 18b (12 mg, 11%) and
19b (120 mg, 49%). In MeCN solution of RB: 5 min irradiation,
column chromatography to yield 18b (36 mg, 12%) and 19b
(115 mg, 47%). In toluene solution of C60: 10 min irradiation,
column chromatography to yield 18b (124 mg, 41%) and 19b
(46 mg, 19%).
Photoreactions of oxygenated solution of 16e and 17
In MeCN solution of DCA: 10 min irradiation, column chro-
matography (EtOAc : hexane ¼ 1 : 5) to yield 18e (65 mg, 21%)
and 19e (77 mg, 31%). In MeCN solution of DCN: 90 min irra-
diation, column chromatography to yield 18e (38 mg, 12%) and
19e (99 mg, 40%). In MeCN solution of RB: 10 min irradiation,
column chromatography to yield 18e (37 mg, 12%) and 19e
(111 mg, 45%). In toluene solution of C60: 20 min irradiation,
column chromatography to yield 18e (107 mg, 35%) and 19e
(49 mg, 20%).
18b (yellow liq). 1H-NMR 0.27 (s, 9H), 1.19 (t, 3H, J ¼ 6.9 Hz),
2.28 (s, 3H), 3.79 (s, 3H), 3.87 (s, 3H), 4.12 (q, 2H, J ¼ 6.9 Hz),
5.72 (s, 2H), 6.67 (d, 2H, J ¼ 7.8 Hz), 7.06 (d, 2H, J ¼ 7.8 Hz); 13C-
NMR 1.1, 13.9, 21.1, 51.3, 51.8, 52.5, 61.0, 122.5, 124.3, 125.0,
126.5, 129.4, 135.3, 136.8, 145.5, 159.5, 164.4, 166.8; HRMS
(FAB) m/z 432.1841 (M + 1, C22H30NO6Si requires 432.1842).
19b (yellow liq). 1H-NMR 1.23 (t, 3H, J ¼ 7.2 Hz), 2.31 (s, 3H),
3.61 (s, 3H), 3.69 (s, 2H), 3.90 (s, 3H), 4.16 (q, 1H, J ¼ 7.2 Hz),
4.34 (s, 2H), 4.70 (s, 1H), 7.13 (s, 4H); 13C-NMR 14.3, 21.3, 50.0,
51.1, 53.2, 55.0, 61.7, 86.8, 128.2, 129.7, 131.8, 138.2, 154.5,
165.9, 167.9, 168.4; HRMS (FAB) m/z 350.1603 (M + 1,
18e (yellow liq). 1H-NMR 0.27 (s, 9H), 1.19 (t, 3H, J ¼ 7.2 Hz),
3.79 (s, 3H), 3.87 (s, 3H), 4.13 (q, 2H, J ¼ 7.2 Hz), 5.72 (s, 2H),
6.73–6.78 (m, 2H), 6.96 (t, 2H, J ¼ 8.7 Hz); 13C-NMR 1.3, 14.0,
51.0, 51.9, 52.6, 61.2, 115.8 (d, J ¼ 21.6 Hz), 122.9, 124.2, 126.9
(d, J ¼ 8 Hz), 134.1 (d, J ¼ 3.2 Hz), 145.6, 159.6, 161.8 (d, J ¼
244.1 Hz), 164.3, 166.8; HRMS (FAB) m/z 436.1589 (M + 1, C21-
H27FNO6Si requires 436.1592).
C
18H24NO6 requires 350.1604).
Photoreactions of oxygenated solution of 16c and 17
19e (yellow liq). 1H-NMR 1.14 (t, 3H, J ¼ 6.9 Hz), 3.50 (s, 3H),
3.64 (s, 2H), 3.80 (s, 3H), 4.07 (q, 1H, J ¼ 6.9 Hz), 4.27 (s, 2H),
4.60 (s, 1H), 6.92 (t, 2H, J ¼ 8.7 Hz), 7.14–7.18 (m, 2H); 13C-NMR
13.8, 49.8, 50.6, 52.7, 54.2, 61.3, 86.8, 116.0 (d, J ¼ 21.5 Hz),
129.4 (d, J ¼ 8.2 Hz), 130.4 (d, J ¼ 3.1 Hz), 153.9, 162.8 (d, J ¼
245.6 Hz), 165.4, 167.3, 168.8; HRMS (FAB) m/z 354.1355 (M + 1,
In MeCN solution of DCA: 5 min irradiation, column chroma-
tography (EtOAc : hexane ¼ 1 : 5) to yield 18c (3 mg, 1%) and
19c (121 mg, 48%). In toluene solution of C60: 10 min irradia-
tion, column chromatography to yield 18c (120 mg, 38%) and
19c (40 mg, 16%).
18c (yellow liq). 1H-NMR 0.21 (s, 9H), 1.18 (t, 3H, J ¼ 7.2 Hz),
2.24 (s, 3H), 2.25 (s, 3H), 3.79 (s, 3H), 3.88 (s, 3H), 4.11 (q, 2H, J ¼
7.2 Hz), 5.63 (s, 2H), 5.98 (d, 1H, J ¼ 7.8 Hz), 6.82 (d, 1H, J ¼ 7.8
Hz), 6.95 (s, 1H); 13C-NMR 0.8, 13.8, 18.8, 20.9, 49.9, 51.6, 52.4,
C
17H21FNO6 requires 354.1353).
Photoreactions of oxygenated solution of 16f and 17
60.9, 122.3, 123.5, 124.0, 126.6, 127.1, 130.8, 133.3, 133.8, 136.5, In MeCN solution of DCA: 10 min irradiation, column chro-
145.8, 159.4, 164.2, 166.9; HRMS (EI) m/z 445.1922 (M+, matography (EtOAc : hexane ¼ 1 : 5) to yield 18f (76 mg, 24%)
C
23H31NO6Si requires 445.1921).
and 19f (86 mg, 33%). In toluene solution of C60: 20 min irra-
19c (yellow liq). 1H-NMR 1.23 (t, 3H, J ¼ 6.9 Hz), 2.18 (s, 3H), diation, column chromatography to yield 18f (102 mg, 32%) and
2.27 (s, 3H), 3.60 (s, 3H), 3.63 (s, 2H), 3.88 (s, 3H), 4.15 (q, 1H, J ¼ 19f (55 mg, 21%).
6.9 Hz), 4.33 (s, 2H), 4.76 (s, 1H), 6.94–7.06 (s, 3H); 13C-NMR
18f (yellow liq). 1H-NMR 0.25 (s, 9H), 1.20 (t, 3H, J ¼ 7.2 Hz),
14.1, 18.8, 20.9, 49.4, 50.9, 52.3, 53.0, 61.4, 87.2, 126.9, 128.6, 3.79 (s, 3H), 3.87 (s, 3H), 4.14 (q, 2H, J ¼ 7.2 Hz), 5.71 (s, 2H),
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6.24–6.32 (m, 1H), 6.70–6.84 (m, 2H); 13C-NMR 0.7, 13.7, 46.0, 52.9, 56.3, 84.6, 127.3, 127.8, 128.7, 135.5, 154.9, 166.0, 168.0;
51.6, 52.4, 61.0, 103.7 (t, J ¼ 25.1 Hz), 111.4 (dd, J ¼ 21.2 Hz, 3.7 HRMS (EI) m/z 263.1154 (M+, C14H17NO4 requires 263.1158).
Hz), 121.8 (dd, J ¼ 14.6 Hz, 3.8 Hz), 122.7, 123.9, 126.7, 126.8
(dd, J ¼ 9.7 Hz, 5.6 Hz), 145.6, 158.8 (dd, J ¼ 247.1 Hz, 11.7 Hz),
Photoreactions of oxygenated solution of 21 and 17
159.1, 162.0 (dd, J ¼ 247.1 Hz, 11.6 Hz), 163.9, 166.4; HRMS (EI)
In MeCN solution of DCA: 5 min irradiation, column chroma-
m/z 453.1421 (M+, C21H25F2NO6Si requires 453.1419).
tography (EtOAc : hexane ¼ 1 : 5) to yield 23 (112 mg, 61%). In
19f (yellow liq). 1H-NMR 1.21 (t, 3H, J ¼ 7.2 Hz), 3.57 (s, 3H),
MeCN solution of DCN: 60 min irradiation, column chroma-
3.74 (s, 2H), 3.86 (s, 3H), 4.14 (q, 2H, J ¼ 7.2 Hz), 4.37 (s, 2H),
tography to yield 23 (111 mg, 60%). In MeCN solution of RB:
4.67 (s, 1H), 6.73–6.86 (m, 2H), 7.24–7.31 (m, 1H); 13C-NMR
5 min irradiation, column chromatography to yield 23 (107 mg,
14.0, 48.2 (d, J ¼ 3.8 Hz), 50.8, 50.9, 53.0, 61.6, 87.6, 103.9 (t, J
58%). In toluene solution of C60: 10 min irradiation, column
chromatography to yield 23 (144 mg, 78%).
¼ 25.3 Hz), 111.7 (dd, J ¼ 21.2 Hz, 3.8 Hz), 117.8 (dd, J ¼ 14.3 Hz,
3.7 Hz), 130.65 (dd, J ¼ 9.4 Hz, 5.2 Hz), 153.8, 160.7 (dd, J ¼
248.0 Hz, 11.9 Hz), 162.6 (dd, J ¼ 248.4 Hz, 11.9 Hz), 165.5,
167.4, 167.9; HRMS (EI) m/z 371.1182 (M+, C17H19F2NO6
Conflicts of interest
requires 371.1180).
There are no conict of interest to declare.
Photoreactions of oxygenated solution of 16g and 17
Acknowledgements
In MeCN solution of DCA: 20 min irradiation, column chro-
matography (EtOAc : hexane ¼ 1 : 5) to yield 18g (108 mg, 32%),
This investigation was nancially supported from National
Research Foundation of Korea (NRF-2018R1D1A3B07049687).
19g (64 mg, 22%). In MeCN solution of DCN: 180 min irradia-
tion, column chromatography to yield 18g (71 mg, 21%) and 19g
(90 mg, 32%). In MeCN solution of RB: 30 min irradiation,
column chromatography to yield 18g (36 mg, 11%) and 19g
Notes and references
(115 mg, 41%). In toluene solution of C60: 30 min irradiation,
1 L. Eberson, Electron Transfer Reactions in Organic Chemistry,
column chromatography to yield 18g (105 mg, 31%) and 19g
Springer-Verlag, New York, 1987.
(59 mg, 21%).
2 M. Gratzel, Heterogeneous Photochemical Electron Transfer,
18g (yellow liq). 1H-NMR 0.26 (s, 9H), 1.18 (t, 3H, J ¼ 7.2 Hz),
CRC Press, Boca Raton, Florida, 1989.
3.80 (s, 3H), 3.87 (s, 3H), 4.12 (q, 2H, J ¼ 7.2 Hz), 5.81 (s, 2H),
3 P. S. Mariano and J. L. Stavinoha, in Synthetic Organic
6.91 (d, J ¼ 8.1 Hz), 7.53 (d, J ¼ 8.1 Hz); 13C-NMR 1.0, 13.7, 51.1,
Photochemistry, ed. W. M. Horspool, Plenum Press, New
51.7, 52.4, 61.0, 122.8, 123.8, 125.3, 125.7 (q, J ¼ 3.7 Hz), 126.9,
York, 1984, pp. 145–257.
142.3, 145.5, 159.3, 164.0, 166.5; HRMS (EI) m/z 485.1479 (M+,
4 (a) G. J. Kavarnos and N. J. Turro, Chem. Rev., 1986, 86, 401;
C
22H26F3NO6Si requires 485.1482).
(b) D. H. Evans, Chem. Rev., 1990, 90, 739; (c) M. Julliard and
M. Chanon, Chem. Rev., 1983, 83, 425.
19g (yellow liq). 1H-NMR 1.24 (t, 3H, J ¼ 7.2 Hz), 3.61 (s, 3H),
3.74 (s, 2H), 3.89 (s, 3H), 4.18 (q, 2H, J ¼ 7.2 Hz), 7.38 (d, 2H, J ¼
8.4 Hz), 7.59 (d, 2H, J ¼ 8.4 Hz); 13C-NMR 14.0, 50.7, 51.0, 53.1,
54.7, 61.7, 87.8, 125.8 (q, J ¼ 3.7 Hz), 127.9, 139.0, 153.9, 165.5,
167.4, 168.0; HRMS (EI) m/z 403.1244 (M+, C18H20F3NO6
requires 403.1243).
5 (a) M. Fagnoni, D. Dondi, D. Ravelli and A. Albini, Chem.
Rev., 2007, 107, 2725; (b) N. Hoffmann, J. Photochem.
Photobiol., C, 2008, 9, 43; (c) D. Ravelli, M. Fagnoni and
A. Albini, Chem. Soc. Rev., 2013, 42, 97; (d) S. Fukuzumi
and K. Ohkubo, Org. Biomol. Chem., 2014, 12, 6059.
6 N. J. Turro, Principle of Molecular Photochemistry: An
Photoreactions of oxygenated solution of 20 and 17
Introduction,
University
Science
Books,
Sausalito,
In MeCN solution of DCA: 10 min irradiation, column chro-
matography (EtOAc : hexane ¼ 1 : 5) to yield 19a (29 mg, 10%),
22 (62 mg, 34%) and 23 (18 mg, 10%). In MeCN solution of DCN:
60 min irradiation, column chromatography to yield 19a
(41 mg, 14%), 22 (58 mg, 32%) and 23 (18 mg, 10%). In MeCN
solution of RB: 10 min irradiation, column chromatography to
yield 19a (50 mg, 17%), 22 (53 mg, 29%) and 23 (20 mg, 11%). In
toluene solution of C60: 30 min irradiation, column chroma-
tography to yield 19a (123 mg, 42%) and 22 (34 mg, 19%).
22 (yellow liq). 1H-NMR 1.25 (t, 3H, J ¼ 7.2 Hz), 2.91 (s, 3H),
3.60 (s, 3H), 3.81 (s, 2H), 3.87 (s, 3H), 4.18 (q, 2H, J ¼ 7.2 Hz),
4.65 (s, 1H); 13C-NMR 14.1, 39.2, 50.9, 53.0, 54.0, 61.6, 86.9,
154.3, 165.7, 167.7, 168.4; HRMS (EI) m/z 259.1052 (M+,
California, 2009.
7 (a) J. P. Dinnocenzo and T. E. Banach, J. Am. Chem. Soc.,
1989, 111, 8646; (b) X. Zhang, S.-R. Yeh, S. Hong,
M. Freccero, A. Albini, D. E. Falvey and P. S. Mariano, J.
Am. Chem. Soc., 1994, 116, 4211; (c) D. D. M. Wayner,
J. J. Dannenberg and D. Griller, Chem. Phys. Lett., 1986,
131, 189.
8 (a) U. C. Yoon and P. S. Mariano, Acc. Chem. Res., 1992, 25,
233; (b) Y. Miyake, K. Nakajima and Y. Nishibayashi, J. Am.
Chem. Soc., 2012, 134, 3338; (c) D. W. Cho, U. C. Yoon and
P. S. Mariano, Acc. Chem. Res., 2011, 44, 204.
9 (a) U. C. Yoon, Y. X. Jin, S. W. Oh, C. H. Park, J. H. Park,
C. F. Campana, X. Cai, E. N. Duesler and P. S. Mariano, J.
Am. Chem. Soc., 2003, 125, 10664; (b) Z. Su, P. S. Mariano,
D. E. Falvey, Y. C. Yoon and S. W. Oh, J. Am. Chem. Soc.,
1998, 120, 10676; (c) D. W. Cho, J. H. Choi, S. W. Oh,
C
11H17NO6 requires 259.1056).
23 (yellow liq). 1H-NMR 2.72 (s, 3H), 3.61 (s, 3H), 3.90 (s, 3H),
4.27 (s, 2H), 4.65 (s, 1H), 7.19–7.34 (m, 5H); 13C-NMR 36.8, 50.7,
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RSC Advances
Paper
C. Quan, U. C. Yoon, R. Wang, S. Yang and P. S. Mariano, J.
Am. Chem. Soc., 2008, 130, 2276.
2011, 47, 9615; (c) A. Fujiya, M. Tanaka, E. Yamaguchi,
N. Tada and A. Itoh, J. Org. Chem., 2016, 81, 7262.
10 (a) K. Nakajima, M. Kitagawa, Y. Ashida, Y. Miyake and 19 S. H. Lim, D. W. Cho and P. S. Mariano, Heterocycles, 2016,
Y. Nishibayashi, Chem. Commun., 2014, 50, 8900; (b) 93, 202.
X. Dai, D. Cheng, B. Guan, W. Mao, X. Xu and X. Li, J. Org. 20 (a) J. W. Arbogast, A. O. Darmanyan, C. S. Foote, Y. Rubin,
Chem., 2014, 79, 7212; (c) S. H. Lim, J. Yi, G. M. Moon,
C. S. Ra, K. Nahm, D. W. Cho, K. Kim, T. G. Hyung,
U. C. Yoon, G. Y. Lee, S. Kim, J. Kim and P. S. Mariano, J.
Org. Chem., 2014, 79, 6946.
F. N. Diederich, M. M. Alvarez, S. J. Anz and R. L. Whetten,
J. Phys. Chem., 1991, 95, 11; (b) R. J. Sension,
C. M. Phillips, A. Z. Szarka, W. J. Romanow, A. R. McGhie,
J. P. MaCauley Jr, A. B. Smith III and R. M. Hochstrasser, J.
Phys. Chem., 1991, 95, 6075; (c) R. Bernstein and
C. S. Foote, J. Phys. Chem. A, 1999, 103, 7244.
11 (a) V. Malatesta and K. U. Ingold, J. Am. Chem. Soc., 1973, 95,
6400; (b) F. D. Lewis, Acc. Chem. Res., 1979, 12, 152; (c)
S. G. Cohen and H. M. Chao, J. Am. Chem. Soc., 1968, 90, 21 (a) Y.-Q. Zou, L.-Q. Lu, L. Fu, N.-J. Chang, J. Rong, J.-R. Chen
165; (d) M. Bellas, D. Bryce-Smith, M. T. Clarke, A. Gilbert,
G. Klunkin, S. Krestonosich, C. Manning and S. J. Wilson,
J. Chem. Soc., Perkin Trans. 1, 1977, 23, 2571.
and W.-J. Xiao, Angew. Chem., Int. Ed., 2011, 50, 7171; (b)
E. E. Maroto, M. Izquierdo, S. Reboredo, J. Marco-
Martinez, S. Filippone and N. Martin, Acc. Chem. Res.,
2014, 47, 2660.
12 (a) N. Paillous and A. Lattes, Tetrahedron Lett., 1971, 12,
4945; (b) K. Krowicki, N. Paillous, M. Riviere and A. Lattes, 22 (a) S. H. Lim, H. C. Jeong, Y. Sohn, Y.-I. Kim, D. W. Cho,
J. Heterocycl. Chem., 1976, 13, 555; (c) K. Koch-Pomeranz,
H. J. Hansen and H. Schmid, Helv. Chim. Acta, 1975, 58, 178.
13 (a) V. D. Parker and M. Tilset, J. Am. Chem. Soc., 1991, 113,
8778; (b) W. Xu, X. M. Zhang and P. S. Mariano, J. Am.
Chem. Soc., 1991, 113, 8863.
H.-J. Woo, I.-S. Shin, U. C. Yoon and P. S. Mariano, J. Org.
Chem., 2016, 81, 2460; (b) H. C. Jeong, S. H. Lim,
D. W. Cho, S. H. Kim and P. S. Mariano, Org. Biomol.
Chem., 2016, 14, 10502.
23 Y. Wang, O. Haze, J. P. Dinnocenzo, S. Farid, R. S. Farid and
I. R. Gould, J. Org. Chem., 2007, 72, 6970.
14 (a) F. D. Lewis and T. L. Ho, J. Org. Chem., 1981, 46, 1077; (b)
X. M. Zhang and P. S. Mariano, J. Org. Chem., 1991, 56, 1655; 24 (a) T. Shen, Z.-G. Zhao, Q. Yu and H.-J. Xu, J. Photochem.
(c) Y. Miyake, Y. Ashida, K. Nakajima and Y. Nishibayashi,
Chem. Commun., 2012, 48, 6966; (d) P. Kohls, D. Jadhav,
G. Pandey and O. Reiser, Org. Lett., 2012, 14, 672.
Photobiol., A, 1989, 47, 203; (b) C. R. Lambert and
I. E. Kochevar, Photochem. Photobiol., 1997, 66, 15.
25 (a) K. Mikami, S. Matsumoto, A. Ishida, S. Takamuku,
T. Suenobu and S. Fukuzumi, J. Am. Chem. Soc., 1995, 117,
11134; (b) D. Dubois, K. M. Kadish, S. Flanagan and
L. J. Wilson, J. Am. Chem. Soc., 1991, 113, 7773; (c)
J. W. Arbogast, C. S. Foote and M. Kao, J. Am. Chem. Soc.,
1992, 114, 2277.
15 (a) N. A. Romero and D. A. Nicewicz, Chem. Rev., 2016, 116,
10075; (b) L. R. Espelt, E. M. Wiensch and T. P. Yoon, J.
Org. Chem., 2013, 78, 4107.
16 (a) T. J. Burkey, A. L. Castelhano, D. Griller and F. P. Lossing,
J. Am. Chem. Soc., 1983, 105, 4701; (b) D. D. M. Wayner and
D. Griller, J. Am. Chem. Soc., 1985, 107, 7764.
26 K. Hu, M. E. Niyazymbetov and D. H. Evans, J. Electroanal.
Chem., 1995, 396, 457.
17 (a) C. J. Li, Acc. Chem. Res., 2009, 42, 335; (b) G. Pandey,
P. Y. Reddy and U. T. Bhalerao, Tetrahedron Lett., 1991, 32, 27 (a) D. C. Dobrowolski, P. R. Ogllby and C. S. Foote, J. Phys.
5147; (c) Y. L. Chow, W. C. Danen, S. F. Nelsen and
D. H. Rosenblatt, Chem. Rev., 1978, 78, 243; (d)
Chem., 1993, 87, 2261; (b) R. C. Kanner and C. S. Foote, J.
Am. Chem. Soc., 1992, 114, 678.
S. I. Murahashi, Angew. Chem., Int. Ed. Engl., 1995, 34, 2443. 28 (a) L. Shi and W. Xia, Chem. Soc. Rev., 2012, 41, 7687; (b)
18 (a) Y.-Q. Zou, L.-Q. Lu, L. Fu, N.-J. Chang, J. Rong, J.-R. Chen
and W.-J. Xiao, Angew. Chem., Int. Ed., 2011, 50, 7171; (b)
M. Rueping, D. Leonori and T. Poisson, Chem. Commun.,
J. M. R. Narayanam and C. R. J. Stephenson, Chem. Soc.
Rev., 2011, 40, 102.
5648 | RSC Adv., 2019, 9, 5639–5648
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